Sheets of multi-layered materials have been used to prepare seals to seal the openings of containers from air and contaminants. A typical seal consists of a sheet of metal foil bearing a layer of heat sealable material for bonding the foil to the lip of a container, for example, a plastic tray type food package, a bottle or a blister package. When used to seal a bottle, the seal is typically die-cut from sheets of a multi-layered material and sealed by means of heat to the lip of the container. When the containers are blister type packages, that is a cavity in a larger sheet, the seals are usually in the form a sheet cover that extends across the blister package opening or across an array of a plurality of such blisters. Plastic food containing trays are widely used to contain meals that require cooking or heating, and are usually deeper than the typical blister packages, having a top lip that provides a small surface on which is removably adhered a lid, sealing the package.
Seals and lids can be constructed to have a tab that extends outwardly from the periphery of the seal so that a user can grasp the tab to aid in removing the seal from the container. In an alternate arrangement when the seal is used with a blister type package the seal may include a scored edge that may be lifted to provide a grip to remove the seal.
The seal may be external, as in the case of a blister package where the seal is the outermost element of the package or it may be an inner seal such as is found in containers having a threaded neck, for use with a threaded cap. The inner seal is usually die-cut from the aforementioned multi-layered sheet of metal foil bearing a layer of heat sealable material. The inner seal is then inserted into the cap, and the cap is twisted onto the neck of the container. The capped container is then heated by means of induction to heat the aluminum foil and thereby melt the layer of heat sealable material so that the inner seal will be bonded to the container.
Whether used to seal blister type packages or the opening of a bottle like container it is desirable that the seals exhibit resistance to tearing, including biting. The latter is particularly important when it is desired to prevent children from accessing the package contents.
Materials used for making such peelable seals include biaxially oriented polyester film such as Mylar® with an amorphous polyester heat seal layer coated thereon. The seal may further comprise printed indicia on an exposed surface of the Mylar® sheet. The indicia may further be covered by a second Mylar® polyester film layer. In an alternative arrangement a second Mylar® film layer having reversed printed indicia on one surface is adhered to the upper surface of the seal so that the printed indicia appear properly oriented and protected by the Mylar® film.
Other known structures include Mylar® RL43 coated with an ethylene vinyl acetate (EVA) heat activated seal to provide a peelable seal. For the reasons discussed earlier, lid seals must provide resistance to tearing particularly during the pealing step, be reasonably flexible, be resistant to water or at least to moisture, be heat resistant when used to seal containers containing materials that need heating such as food, accept printing on at least an exposed surface thereof, and be inexpensive to produce. In addition, it is desirable that such seals be made so that they present few if any disposal problems, and be environmentally friendly.
U.S. Pat. No. 5,372,268 issued to Han and assigned to the 3M Company discloses a multilayer structure for use as a heat seal. Such structure comprises (a) a layer of sealing material for bonding the seal to the container, (b) a layer that is substantially impermeable to air and moisture, and (c) a layer of reinforcing material. The layer of sealing material can be formed from any suitable adhesive, such as, for example, a heat sealable film, a hot melt adhesive, a tack-free pressure-activated adhesive, or a shear-activated adhesive. The impermeable layer can be formed from polymeric film, e.g., poly(vinyl chloride), or metal foil, e.g., aluminum foil. The reinforcing layer is a multi-ply film consisting of at least two plies of monoaxially oriented films that are laminated together with the strong direction of one ply crossing the weak direction of another ply to impart equal strength in all directions such that the tear and tensile strengths are increased in the machine direction, the cross direction, and in a diagonal direction.
The use of chemically dissimilar materials to form the seal as disclosed in the prior art tends to increase the thickness and stiffness of the final product, as well as the manufacturing costs by increasing the number of material layers used and the complexity of the manufacturing steps when one attempts to satisfy all of the above stated desirable attributes of a seal material. In particular, disposal of waste material generated during the manufacturing and use of the webs used to form the seals presents a disposal problem. Such problem is all the more vexing when the web produced exhibits the desirable characteristics of high resistance to tearing and imperviousness to humidity or water. There is, therefore, still need for a material for use as a sealing lid for containers that is relatively easy and inexpensive to fabricate, exhibits the high degree of tear resistance that is desirable for such lids, and which may be conveniently disposed of when necessary.